Primary rock magnetism for the Wenchuan earthquake fault zone at Jiulong outcrop, Sichuan Province, China

摘要

Primary rock magnetism analysis was performed on samples from the Jiulong outcrop across the Anxian– Guanxian fault of the 2008 Wenchuan Earthquake rupture zone. The protolith of hanging wall of this outcrop is the upper Triassic sediments, which formed the fault breccia and gouge by repeated large earthquakes. The footwall of this outcrop contains Jurassic grayish-green and dark-purple sandstones. The average magnetic susceptibility value of the gouge is slightly less than that of potential protolith. Based on the primary rock magnetism, the main magnetic carriers are Fe-sulfides for the gouge, magnetite for the fault breccia, and magnetite and hematite for the Jurassic grayish-green and dark-purple sandstones. Possibly during or after repeated large earthquakes (just like the 2008Mw7.9 Wenchuan Earthquake), it transformed the magnetic mineral frommagnetite to Fe-sulfides by low thermal decomposition processes along the Anxian–Guanxian earthquake fault, which induces the slightly less average magnetic susceptibility values of the gouge than that of potential protolith. If this magnetic mineral changed only because of repeated large earthquake process, the heating by low velocity seismic slip friction and seismic fluid could possibly have been less than 300 ℃. If this magnetic mineral of the Anxian–Guanxian earthquake fault is only induced after repeated large earthquakes, the earth surface process acts an important role for the magnetic mineral change. More other further studies should be done to verify the primary magnetic mineral phase change and discriminate the time of this magnetic mineral variation.